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Ultrasonic full-body inspection system for casing tubes

Jul 06, 2006

An ultrasonic inspection system for the full-body inspection of casing tubes was delivered by NDT Systems & Services AG to China's leading steel works in 2005.The inspection system has been designed for the fully automated ultrasonic inspection of ERW and seamless tubes with diameters ranging from 140 to 400 mm and wall thicknesses ranging from 3 to 25 mm.

For the full-body inspection, the tube is spirally guided through several inspection basins where probes are installed at different angles of incidence. Coupling is achieved by utilizing the partial immersion technique (PIT), i.e. the bottom of the tube is immersed some millimeters into the overflowing water basins.

As shown in Figure 1, the point of incidence for the probes and for each tube diameter is located in the 6 o'clock position. As a result, the angle of incidence almost remains the same so that the angles of the probes need not be adjusted to the diameter of the tube to be inspected. When changing to a different tube size, only the distance between the probes and the bottom of the tube, which amounts to some millimeters, has to remain constant. This is achieved by mechanically adjusting the guiding rolls which are located on both sides of the inspection basins (Figure 2).
In order to obtain a high inspection velocity, the number of probes has been chosen for a helical track width of 150 mm. A total of 60 probes is used.
  • detection of longitudinal defects:
    2 x 2 probe arrays consisting of 8 elements each, partitioned on two basins; adjustable incidence angle in limits
  • detection of transverse defects:
    24 single probes, partitioned on two basins; fixed incidence angle
  • measurement of wall thickness:
    4 single probes, partitioned on two basins; vertical incidence

In order to obtain 100% inspection coverage and to safely detect the reference flaws, a 10% overlap of the neighboring scanning tracks has been considered for the effective tube feed of > 135 mm/revolution. To ensure a stable coupling, the circumferential speed of the tube is limited. Depending on the tube diameter, inspection velocities of a maximum of 0.35 m/s are achieved.
Angularly adjustable pairs of rolls driven by motors are used for the helical transport of the tubes (Figure 3). The angle setting depends on the tube diameter and ranges from 7 to 20°. Since the reproducibility of the inspection results considerably depends on a constant tube movement, the accuracy of the angular adjustment must be < 0.5°.

These drive units are distributed over the complete inspection line which is subdivided into the following sections:
  • kick-in unit (feeding the tubes into the inspection line)
  • inspection area (ultrasonic inspection line)
  • kick-out unit (outlet/sorting of the inspected tubes)

The distance between the drive units has been selected such that the shortest tube is always supported by three pairs of rolls. Considering an admissible deviation from the straight line of 0.2% (according to API 5CT), the drive units in the inspection line area have additionally been equipped with hold-down rolls. This allows for a smoother movement of the tubes through the inspection basins.
Each inspection mechanics consists of two identically designed inspection basins (Figure 4). The inspection basins are lifted towards the tube and lowered by a sensor-controlled pneumatic system. It ensures that the inspection basins are lifted only when the leading edge of the tube has reached a defined position. The vertical and horizontal transverse movement of the helically transported tube is transmitted to the inspection basins by means of guiding rolls. Due to the pneumatic attachment and their elastic suspension they are able to follow the inevitable transverse movement of the tube. If the transverse movement of the tube is too intense, the inspection basins are automatically lowered and the inspection is aborted.

Due to the small immersion depth and the helical movement, coupling water is prevented from entering the tube. When the trailing edge has reached the corresponding probe, the ultrasonic electronics is automatically switched off in order to avoid false indications.

In order to avoid the ormation of air bubbles in the inspection basins, a coupling water recirculation loop with stilling basins is employed. Water is supplied into the inspection basins only by utilizing hydrostatic pressure.

The tubes pass the inspection line at a distance of approximately 2 m to each other. For tracking the tubes, the current feed is determined individually for each tube by means of several calibration sections and considered during evaluation through the Data Evaluation Unit (DEUPC). True-to-location in axial direction, spraying guns mark the location of detected flaws on the tube.

For each tube, the inspection results are stored in the form of a strip chart recording of the ultrasonic signal (A-scan), as a C-scan with colored marking of the type and location of the detected flaws as well as in the form of a summarizing tube protocol. Depending on the inspection results, the kick-out unit sorts the inspected tubes according to accepted or rejected.
Table 1: Features of the Automatic Tube Inspection System
inspection technology Partial Immersion Technique
outer tube diameter 140 ... 400 (optional 100 ... 500 mm)
wall thickness 3 ... 25 mm
tube length 4 ... 15 m (typical)
effective helix track 135 mm (including overlap)
number of probes (or elements) per inspection basin 2 x 8 for longitudinal defects + 2 x 1 for wall thickness
12 x 1 for transverse defects
number of ultrasonic channels 60 (multiplexing system)
ratation limit 150 rev/min
circumferential speed 1.0 ... 1.5 m/s
inspection speed 0.35 m/s (depending on th tube diameter
circumfrential pulse density < 0.5 mm
angle of helical track 6 ... 20°
tube conveyance spirally
inspection process continuously (distance of approx. 2 m between two tubes
standards APII 5CT, API 5L
NDT Systems & Services AG is a technology company with its major focus on developing and providing advanced non-destructive ultrasonic testing systems and services for ferrous and non-ferrous metals.

More than 25 ultrasonic inspection systems including EMAT technology provided by the business unit NDT Systems are utilized worldwide for the automated production quality control of heavy plate, strip material and tubes as well as in the fields of rail and automotive industry.

The NDT Services business unit provides global pipeline inspection services. The full range of services includes geometry inspection, metal loss and crack inspections, defect assessments and fitness-for-purpose investigations.


Contact:
Dr.-Ing. Michael Schmeisser
NDT Systems & Services AG
Stutensee, Germany
E-Mail: michael.schmeisser@ndt-ag.de

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Dr.-Ing. Michael Schmeisser

Stutensee (Germany)

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+49 (0)7244 - 7415-0

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michael.schmeisser@ndt-ag.de

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